1. Field of the Invention
The present invention relates to an intake-air cooling device for an internal combustion engine and to an automobile using the same.
2. Description of Related Art
One widely used way of raising the output power of internal combustion engines, such as gasoline engines, is to increase the amount of intake air using a supercharger.
When the intake air is compressed with a supercharger, the temperature of the intake air rises and the inlet temperature to the gasoline engine thus increases, which reduces the charging efficiency and increases the temperature of the air-fuel mixture, which tends to cause knocking.
Methods of suppressing knocking include delaying the ignition timing and enriching the air-fuel ratio of the air-fuel mixture; however, these methods deteriorate the output and the fuel consumption.
Therefore, the intake air sent from the supercharger is usually cooled by using an intercooler in which outside air functions as the cooling source.
One device that has been proposed to effectively perform such cooling is disclosed, for example, in Japanese Unexamined Patent Application, Publication No. SHO-62-153518. It is equipped with a first heat exchanger that cools the intake air from the supercharger by using outside air and a second heat exchanger, downstream of the first heat exchanger, that cools the intake air with refrigerant that circulates in a refrigeration circuit of a vehicle-cabin cooling device.
The intake air temperature is measured at an outlet of the first heat exchanger, and if this intake air temperature exceeds a predetermined value, it is assumed that there is a risk of knocking, and the second heat exchanger is thus operated to cool the intake air. Therefore, knocking can be prevented in the internal combustion engine without delaying the ignition timing of the spark plugs and without enriching the air-fuel ratio. Therefore, it is possible to increase the output of the internal combustion and to improve the fuel consumption.
However, when refrigerant is supplied to the second heat exchanger, the refrigerant used to cool the vehicle cabin is reduced by a corresponding amount. Therefore, if, for example, a prescribed cooling performance is required, it is necessary to increase the performance of a compressor. Because the compressor is driven by the internal combustion engine, the output of the internal combustion engine increases by an amount corresponding to the increase in performance of the compressor. In other words, because the output of the internal combustion engine increases, the fuel consumption increases, and the operating efficiency of the internal combustion engine decreases accordingly.
With the invention disclosed in Japanese Unexamined Patent Application, Publication No. SHO-62-153518, it is determined that there is a risk of knocking when the intake air temperature at the outlet of the first heat exchanger exceeds the predetermined value, in other words, indirect determination. Therefore, this determination occurs at a timing having a certain margin relative to the timing at which knocking actually occurs.
As a result, the intake air is cooled with refrigerant even when it is not necessary to do so, and therefore, the operating efficiency of the internal combustion engine is decreased by a corresponding amount.